1. Field of the Invention
The present invention relates generally to high-speed digital communication systems, and specifically to methods for transmission and reception of signals using multicarrier modulation schemes.
2. Description of the Related Art
The meanings of acronyms and certain terminology used herein are given in Table 1.
TABLE 1ADSLAsymmetric Digital Subscriber LineCOCentral OfficeCPECustomer Premises EquipmentDMTDiscrete Multitone ModulationDSLDigital Subscriber LinesFEQFrequency Domain EqualizerFEXTFar End CrosstalkFFTFast Fourier TransformIFFTInverse Fast Fourier TransformL0Full power mode operationL2A power save mode of operation.NEXTNear End CrosstalkPMDPhysical Media DependentQAMQuadrature Amplitude ModulationSNRSignal-to-noise ratioVDSLVery High Speed Digital Subscriber Line
In recent years multicarrier modulation has become the main commercial modulation scheme in “last mile solutions”, particularly Digital Subscriber Lines (DSL). Multicarrier modulation is also used in various wireless applications. The leading representative for this trend is discrete multitone modulation (DMT), which is successfully employed in both DSL and wireless modems. For example, Asymmetric Digital Subscriber Line (ADSL) uses 256 or 512 tones in a downlink, i.e., from a central office (CO) facility to customer premises equipment (CPE). ADSL standards are described in further detail, for example, in the document, ITU-T Recommendation G.992.3, entitled Series G: Transmission Systems and Media, Digital Systems and Networks; Digital Sections, which is herein incorporated by reference.
In one DMT modulation scheme, used in ADSL, N tones are modulated by quadrature amplitude modulation (QAM), using two-dimensional input frequency-domain symbols. A 2N-point inverse fast Fourier transform (IFFT) then produces a corresponding time-domain symbol, expressed as a real baseband time-domain output signal of 2N real samples in each symbol period. At the receiving side, 2N samples are extracted from the time-domain signal during each symbol period. A fast Fourier transform (FFT) is used to demodulate the signal and recover the original QAM symbols on the N tones.
The number of bits to be encoded by each tone, known as bit loading or bit allocation, is determined by the receiver according to line conditions, which are measured as a function of frequency during a training period. The bit loading value for each tone may take any value from zero up to a preset maximum. The receiver passes a table of these values, known as a bit loading table or bit allocation table, to the transmitter, which thus determines how many bits of the input data stream to allocate to each successive tone in the tone order.
In order to achieve higher bit rates using multicarrier multitone techniques, more tones are needed. For example, DMT has been proposed for Very High Speed Digital Subscriber Line (VDSL) modems using 4096 tones or subcarriers. Since, in multicarrier modulation, each subcarrier sees a different slice of the channel, the modulation parameters used for each subcarrier are typically different. For example, the number of bits that can be conveyed or loaded per symbol, i.e., for a given carrier constellation size, is different for each of the carriers. The number of bits per tone is held in the bit loading table. Moreover, the optimal power allocation among the subcarriers (also known as per carrier gain) is not flat. The gain per tone is held in a gain table. In order for the multicarrier scheme to operate, both the bit allocation table and the gain table are typically computed by a receiver, and transferred from time to time from the receiver to a transmitter. In the case of the large number of tones employed in VDSL, these tables are so large that they burden the multicarrier scheme.
In the case of ADSL, a power save mode (L2 mode) was included in the above-noted ITU-T Recommendation G.992.3. The L2 mode uses an online reconfiguration mechanism to allow the central office and consumer premise equipment to negotiate a lower data rate, which the CPE can achieve using a lower average transmit gain level from the CO, with a new bits and gains table defined by the CPE. A reduction in the average gain level translates into a reduction in the average transmit power, allowing the CO to save power. This scheme provides for a reduction in the power consumption of the central office modem, by at most six dB. There is no provision for reduction of the modem power consumption at the customer site.
The above noted technical obstacles indicate the need for improvements in power saving modes of operation in multicarrier modulation schemes. Previous proposed solutions unfortunately sacrifice channel utilization, e.g. by requiring symbol duplication in the case of VDSL, which reduces bit rate unacceptably. Channel utilization is not sacrificed when the principles of the invention are applied.